This invention relates to braking systems for trailers and, more particularly, to systems for controlling the operation of the brakes of a trailer. Trailers, such as mobile homes, campers, boat trailers and the like, which are towed behind a towing vehicle such as, for instance, an automobile, conventionally employ electrical brakes which are actuated by electrical interconnection with the electrical brake control system of the towing vehicle. Thus, as the driver of the towing vehicle operates the towing vehicle brakes by depressing the brake pedal, the electrical brake system of the trailer is simultaneously operated.
Control systems for electrically controlling the brakes of a trailer are shown in U.S. Pat. No. 3,780,832 and U.S. Pat. No. 4,254,998, which patents are assigned to the assignee of the present invention and which are incorporated herein by reference. In general, the systems described in the above-identified patents operate by electrically interconnecting the pedal-operated brake switch with the trailer brake control system. The switch operates the towing vehicle brake lights, and, therefore, provides an indication that the towing vehicle brake pedal has been depressed. These prior art systems also provide for the gradual application of braking force to the trailer brakes so that, when the brake pedal of the towing vehicle is depressed, the control system automatically and gradually applies power to the trailer brakes. However, these prior art control systems do not provide an indication to the driver of the vehicle of the amount of braking force which is being applied to the trailer brakes. It is, therefore, desired to provide a control circuit wherein a perceptible indication is provided of the braking force which is being applied to the trailer brakes.
Prior art brake control systems have also included short circuit protection whereby, despite actuation of the towing vehicle brake pedal, no braking force will be applied to the trailer brakes when an electrical short circuit or overload condition exists in the trailer braking system and whereby the trailer brake control system is disabled to prevent damage to the electrical circuit components. However, these prior art control systems do not provide an indication to the driver of the towing vehicle that such an overload or short circuit condition has developed. It is, therefore, desired to provide a control system whereby a perceptible, i.e., visible or audible, indication is provided to the driver of the towing vehicle that a short circuit or overload situation has occurred in the electrical circuit of the trailer brake control system.
Another problem with the prior art trailer brake control systems has been that the systems and electronic circuits therefor have been relatively complicated. The electronic circuits conventionally have used discrete circuit components which are arranged in an analog-type of electrical circuit. It is desired to provide a simplified digital type of trailer brake control electrical circuit wherein the trailer brakes are actuated by a train of pulses whose width is varied in accordance with the amount of braking force to be applied to the trailer brakes.
Prior art control systems have also provided for adjustment of the amount of braking force to be applied to the trailer brakes to account for weight variation in trailers. However, in some prior art trailer brake control systems, such adjustment has been effected by means of liquid sensitive switches, such as mercury switches. Such switches are also sensitive to the variation in road grade so that the braking force applied to the trailer brakes would vary as a function of the grade of the road being travelled. It is, therefore, desired to provide a trailer brake control system which is not affected by the up or down grade of a road.